1. Field of the Invention
The present invention relates to thin film resistors, and more particularly but not by way of limitation, to Silicon Chromium (SiCr) thin film resistors having improved temperature coefficients. In one aspect, the present invention relates to a process for producing SiCr thin film resistors having improved temperature coefficients of resistance and enhanced sheet resistance values.
2. Description of the Prior Art
SiCr thin film resistors and processes for fabricating such resistors have heretofore been known. Generally, SiCr thin film resistors have been fabricated by forming a thin film of SiCr on a substrate utilizing a DC or rf sputtering process. The SiCr film formed on a substrate using either a DC or rf sputtering process inherently possesses a high temperature coefficient of resistance and the maximum obtainable sheet resistance for such a film is limited.
Temperature coefficient of resistance (TCR) is a measure of the ability of a film's sheet resistance to change the least when the temperature of the film is increased and decreased. For resistors, the (TCR is determined by measuring resistance at different temperatures and thereafter computing the rate of resistance change per degree of temperature. This parameter, which is expressed in parts per million per degrees Celsius (ppm/.degree. C.) and is generally a function of the film material, film composition and deposition conditions, is extremely difficult to control and can be negative (when the resistance decreases as the temperature increases) or positive (when the resistance increases as the temperature increases).
The sheet resistance of a SiCr film can be enhanced by annealing a deposited film in a nitrogen and oxygen environment. The sheet resistance of the SiCr alloy film can be increased approximately 280% by annealing at a temperature of 475 degrees Celsius for 30 minutes. After the first 30 minutes of annealing, further increases in the sheet resistance of the film are dependent on the temperature. However, increasing the temperature for a SiCr film over 480.degree. C. in an effort to further increase the sheet resistance of the SiCr film is not desirable because the SiCr film decomposes at temperatures greater than 480 degrees Celsius and is thus destroyed.
It has also been proposed to enhance the sheet resistance of a thin film by thinning of the film, that is, to deposit a thinner film on the substrate. However, when one attempts to thin down the film the quality of the film degrades and the film becomes more sensitive to downstream processing steps.
SiCr resistors currently available generally possess a TCR which is too high. This renders such resistors unacceptable for many new circuit designs. New and improved processes for fabricating SiCr resistors can substantially lower TCRs, while at the same time enhancing the sheet resistance of the SiCr film. The present invention is directed to such a process.